Fig. 3.
Spiking activity under uni- and bimodal stimulation. Recording traces (A) and rasterplots (B) show representative examples of responses to moving-object stimuli alone (mov/thick horizontal lines). Asterisks mark stimuli that evoked strong (large asterisk) or moderate (small asterisk) responses. C: the corresponding Gaussian-smoothed peristimulus time histograms illustrate how the same respective cell responded to a stationary polarizer (stat pol only) after precedent polarizer rotation stopped (vertical arrow) at an excitatory (Φmax, blue line), neutral (Φneut, black line), or inhibitory E-vector (Φmin, red line). Response to rotation is shown for the TL2 cell only (row 1); λ: firing rate (spikes/s). While some of the polarized-light responses decline to a rate close to the average prestimulus activity (thin horizontal dashed line), others stabilize at a residual plateau (excitatory responses of the TL2 cell and the excitatory responses of TB1 and CPU1 in rows 3 and 4). Occasionally, both cases seem to occur in the same cell (TB1, row 3). In contrast, the activity of the CL1 cell (row 2) following stop of polarizer rotation is marked by slow changes in firing rate both at the inhibitory and neutral E-vector. After stabilization of neural activity the moving object was presented while the presentation of stationary polarized light continued (pol and mov; thick black horizontal bars). With respect to moving object stimuli TL2 and CL1 cells behaved rather stereotypically, with no responsiveness to moving objects alone and combined stimulation in TL2 and a generalized inhibitory response to both scenarios involving the moving object in CL1. By contrast, two different response behaviors were observed in TB1 cells. TB1 cells responsive to the moving object alone (A, row 3, large asterisk) responded stereotypically, i.e., E-vector independently and in the same manner under combined stimulation (C, pol and mov). In TB1 cells unresponsive to the moving object alone (row 4), the combined stimulation triggered a regain of full compass responses, bringing back the unadapted levels of inhibitory or excitatory E-vector responses. The same dichotomy was observed in CPU cells. CPU1/2 neurons that were inhibited during object movement alone (row 5, A and B) maintained inhibitory responses during combined polarizer/moving object stimulation as illustrated for a CPU1 neuron in C. CPU1/2 neurons unresponsive to the moving object alone, however, showed disadaptation of the compass responses when stimulated with a moving object during concurrent presentation of the stationary polarizer as illustrated for a CPU2 neuron (row 6).